I assume this is a blue-phase sergeant major. But, look at the front dorsal-fin; it looks different than any I have seen on photos of sergeant majors. It was feisty, and I assume it was protecting eggs. I mention that in case its behavior and fin shape and placement are in any way related.
Can you explain the unusual fin shape and placement?
The photo was taken April 16, 2009 at Molasses Reef, Key Largo, 19 feet deep, visibility 25 feet, bottom temperature 76 degrees Fahrenheit.